“Now is the time” is to tighten sanctions on Iran, according to Sen. Saxby Chambliss. Chambliss, along with several other Republicans and at least one Democrat (Chuck Schumer), have signaled their intent to hit Iran with new economic punishments in spite of the just-inked interim deal relaxing sanctions in exchange for nuclear concessions.

Tehran has already said any new sanctions would kill the deal, and indeed, putting an additional sanctions regime on Iran would violate its terms. But Congress’s actions may have more desperate consequences: imposing new sanctions now could kill Iranian-American negotiations in perpetuity, forcing a terrible choice between strikes and a nuclear-armed Iran.

To see why, it’s important to understand how the research on how economic sanctions work. A relatively optimistic analysis of 115 sanction regimes found they succeeded about a third of the time; a more recent study of the same dataset suggested the number was closer to four percent. “It would be difﬁcult to ﬁnd any proposition in the international relations literature more widely accepted than those belittling the utility of economic techniques of statecraft,” Princeton Professor David Baldwin writes (Baldwin himself thinks sanctions “work” in a certain oblique sense, but not usually for the stated purpose of forcing another state to make policy concessions.)

In the rare cases that they do work, it’s because the target state’s leaders have made a political decision that suffering sanctions aren’t worth it anymore. Jean-Marc F. Blanchard and Norrin M. Ripsman, in a book summarizing two decades of cooperative research on economic statecraft, argue that sanctions work when the “(1)they manifest themselves as political costs and incentives, rather than merely economic costs, and (2) political costs and incentives outweigh the the target state’s political interest in maintaining the status quo.”

The distinction between “political” and mere “economic” costs is important. International sanctions can do as much damage to the Iranian economy as international policymakers like, but so long as the Iranian leadership values the nuclear program more than an intact economy, it won’t matter. That’s why economic sanctions often fail: they demand political concessions like regime change that the sanctioned state will never give up for any economic price. Some things aren’t for sale.

Though for years many believed Iran’s nuclear program was one of those priceless things in the minds of Iran’s leaders, it’s clear from the interim deal that the regime is reevaluating the political costs and benefits of a nuclear program. The trick now is to convince the Iranians that they have more to gain from concessions in exchange for sanctions relief than from returning to full-bore nuclear enrichment.

That balance depends crucially on what Duke’s Bruce Jentleson calls “reciprocity,” defined as “an explicit or at least mutually tacit understanding of linkage between the coercer’s carrots and the target’s concessions.” Reciprocity’s central importance make intuitive sense: why would Iran agree to give up its nuclear program without believing that America and its partners would actually relax sanctions, and vice versa? Indeed, Jentleson’s history of Libya’s WMD program shows that interim negotiations and smaller tit-for-tat concessions built the kind of reciprocity necessary for a final nuclear agreement between Libya and the international community.

New Congressional sanctions have the power to shatter U.S.-Iran reciprocity. If, just after agreeing to relax sanctions, the Senate overrode the President and imposed new ones, Iran would have zero reason to believe that any agreement with the United States would hold up in Congress. That’s not just an afterthought: the entirety of the President’s power to unilaterally relax sanctions comes from national security waiversin sanctions legislation, which means Congress could torpedo any deal made by any President at any time.

Iranian policymakers payattention to American domestic politics. If they come to believe that Congress will kill any deal the Iranians find acceptable, then they’ll have no reason to try to a final status agreement with the United States. Absent reciprocity, the only options are war or nuclear Iran.

Some might argue that if the Iranians are all of a sudden worried about the damage to their economy from sanctions, then wouldn’t more pain simply cause them to cut a deal on more favorable terms? It’s not like Congress is saying “sanctions forever;” more like, “sanctions until a better deal.”

Not so fast. First of all, the sanctions the Senate would consider would most likely be something like the oil export sanctions the House passed this summer. These are “broad-based” trade sanctions targeted at the Iranian economy, rather than “smart” sanctions targeted at Iran’s military, economic, and political elite.

Broad-based sanctions have an especially poor track record. Using the same 115 sanction database, political scientists Ella Shagabutdinova and Jeffrey Berejikian compared the success rates of broad-based and smart sanctions. They found that smart sanctions were 19 percent more likely to bring about the sanctioner’s best-case scenario than pure trade sanctions, suggesting trade sanctions on their own are relatively ineffective. So the addition of one more trade sanction to the already extensive regime of broad-based and smart sanctions punishing Iranians is unlikely to be a tipping point convincing Iranians to acede to U.S.’s most maximalist terms.

Second, and more fundamentally, the pro-more sanctions argument betrays a fundamental failure to grasp the logic of sanctions. If the cost/benefit calculation associated with sanctions really is political rather than purely economic, then more pain itself does not matter unless accompanied by an attendant shift in Iran’s political cost/benefit analysis. Iran didn’t come to the table purely because of increased economic pain; it was also a calculation made after change in leadership and months of backchannel negotiations between the U.S. and Iran built up reciprocity between the two sides that suggested a political opening for a mutually acceptable deal. Congress undermining American diplomats would wreck Iran’s already-tentative faith in that process, perversely making the economic pain inflicted by sanctions less likely to function as an incentive for Iran to negotiate.

Now, one could say that no deal is better than any deal we could get that would be acceptable to Iran. But that’s an argument for either containing a nuclear Iran or, more likely, war. Anyone seriously committed to a negotiated deal should oppose a push for more Congressional sanctions now.